Anatomical methods which depend upon the anterograde axonal transport of isotopically labeled neuronal proteins or the retrograde axonal transport of the enzyme, horseradish peroxidase, have been used to elucidate the relationships between the reticular complex and the dorsal thalamus and cerebral cortex. Injections of tritiated amino acids in the dorsal thalamus or cerebral cortex in rats, cats and monkeys, show that as the bundles of thalamo-cortical and cortico-thalamic fibers joining a particular dorsal thalamic nucleus to its associated area of the cerebral cortex traverse the reticular complex, they each give rise to a dense zone of terminals occupying a sector of the reticular complex which is relatively constant for that dorsal thalamic nucleus and cortical area. However, because of the wide extent of the dendritic fields of the reticular cells and the degree of overlap between the sectors of the complex subtended by adjacent dorsal thalamic nuclei and adjacent cortical areas, it is likely that the reticular complex samples thalamo-cortical and cortico-thalamic activity in a somewhat unspecific manner. Fibers passing to the reticular complex from the intralaminar nuclei of the thalamus appear to be associated with the projection from the intralaminar nuclei to the striatum. Injections of tritiated amino acids in the reticular complex itself and injections of horseradish peroxidase in various other parts of the brain show that the only efferent pathway from the reticular complex terminates in the nuclei of the dorsal thalamus. The reticular complex does not appear to send fibers to other components of the ventral thalamus nor to the cerebral cortex.